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Chapter 16 Earthquakes Notes (3 classes notes + 1 class lab + 1 class Xword & review) Objectives: 1. Define earthquake. p. 441 2. Explain the causes of earthquakes. pp. 441-443 3. Define foreshock and aftershock. p. 443 4. Explain how a seismograph works. p. 444-445 5. Compare the properties of earthquake waves. pp. 445-447 6. Compare the Richter and Mercalli scales. pp. 451-453 7. Locate the epicenter of an earthquake given appropriate seismographic data. pp. 447-449 and Core Lab #5 An earthquake is the vibration of the earth produced by the rapid release of energy. They are most often caused by slippage along large fractures in the earth's crust called faults. The energy radiates in the form of waves in all directions from its source, called the focus of the earthquake (see Figure 16.3 p. 442 text). This focus is located under the earth's crust. The location on the earth's surface directly above the focus is called the epicenter (see Figure 16.3 p. 442 text). In North America, the most famous earthquake region is along the San Andreas Fault. See animations here & here How Earthquakes Form (see Figure 16.5 p. 443 text) Tectonic forces (the forces that move the roughly 20 plates around) slowly deform the crust on both sides of a fault (see Figure 16.5B p. 443 text). This causes the rocks to bend and gain elastic energy. Eventually, enough force will build up to overcome the forces which hold the rocks together and slippage (see Figure 16.5C p. 443 text) occurs at the weakest point along the fault (focus). This movement will cause other points along the fault to come under stress and additional slippage will occur until all the built up elastic energy is 1 released (see Figure 16.5D p. 443 text). This slippage will allow the rock to return to its original shape and it is the vibration that occurs as the rock returns to its original shape that we call an earthquake. The returning of the rock to its undeformed state is called elastic rebound, as the rock behaves much like an elastic band which is stretched and then returns to its original shape when released. See animations here, here, here, and here Sample Exam Question 1. After an earthquake, what happens to the rocks on either side of the fault? a) They return to their unstrained positions and retain their original shapes. b) They are metamorphosed by the extreme heat and pressure. c) They are melted to form new igneous rocks at the fault boundary. d) They remained stretched to their breaking point. 2. Earthquakes commonly occur at plate boundaries. With reference to elastic rebound, what causes an earthquake? Foreshocks – smaller earthquakes that occur before a major earthquake. These can help predict when a major earthquake will occur. Aftershocks – smaller earthquakes that occur after a major earthquake. The Location of Earthquakes Nearly all earthquakes (95%) are found in a small number of earthquake belts around the globe (see Figure 16.13 p. 449). The largest of these runs around the outer edge of the Pacific Ocean. Another belt runs north of the Mediterranean Sea through the Himalayas. A third runs down the middle of the Atlantic Ocean. 2 Note that most earthquakes occur at divergent, convergent and transform plate boundaries. Also note that not all quakes occur at these boundaries (see Figure 16.A, p. 450). Earthquake Depths Earthquakes are classified by how far below the surface the earthquake occurs (see Figure 16.14, p. 451). i. Those that occur less than 70km under the surface are called shallow focus earthquakes. These usually cause the most damage. ii. Those that occur between 70km and 300km under the surface are called intermediate focus earthquakes. iii. Those that occur more than 300km under the surface are called deep focus earthquakes. It was discovered that in subduction zones, the depth of the earthquake increased as you moved farther from the trench. These regions were named Wadati-Benioff zones after the scientists who first studied them extensively. The depth of the focus of an earthquake depends on the type of plate boundary at which it occurs. Divergent and transform boundaries are typically the location of shallow focus earthquakes whereas shallow, intermediate, and deep focus earthquakes can occur at convergent boundaries. How a Seismograph Works Seismology is the study of earthquake waves. Seismographs are instruments that recode earthquake waves (see Figure 16.7 and Figure 16.8 p. 445 text). A seismograph consists of a weight that is freely suspended from a support that is attached to the bedrock. When an earthquake occurs, the bedrock and the support move but the weight 3 remains stationary because of its inertia (Newton's First Law of Motion or the Law of Inertia). The movement of the bedrock in relation to the stationary weight is recorded on a rotating drum which contains the paper. This record of the ground's motion is called a seismogram (see Figure 16.10 p. 447 text). See animations here, here, and here Seismic waves – Waves that are created by an earthquake and that carry the energy created by the earthquake. Types of Seismic (Earthquake) Waves Seismograms reveal that there are two main types of seismic waves. 1. Surface Waves (Long (L) Waves) - those that travel along the earth's solid outer layer. These are the slowest type of seismic waves. Surface waves cause the ground either to move from side to side horizontally, perpendicular to the path of the wave itself (see Figure 16.9C, p. 446) or to move in an elliptical motion (see Figure 16.9D, p. 446). 2. Body Waves - those that travel through the earth's interior. Body waves are further divided into two types, depending on how they move through the earth. i. Primary Waves (Push-Pull (P) Waves) - these are compression waves which transmit energy much like air transmits sound wave energy. In a compression wave, the particles of the medium move horizontally and parallel to the direction in which the wave moves (see Figure 16.9A p. 446 text). P waves can travel through solids, liquids and gases. P waves travel fastest through the earth, up to 6km/s. See animations here, here, here, and here ii. Secondary Waves (Shear (S) Waves) - these are transverse waves which transmit energy much like a rope transmits energy using a wave. In a transverse wave, the particles of the medium move vertically or horizontally perpendicular to the direction in which the wave moves (see Figure 16.9B p. 446 text). S waves can travel through solids only and not through liquids and gases. S waves are 4 slower (3.6km/s) than P waves but faster than surface waves. See animations here, here, here, and here Sample Exam Question Explain how S waves differ from P waves. Activity: Complete the table below. Name Other Name Surface Wave (L Wave) Long Wave Medium that Wave will Travel Through Particle Motion Slowest Horizontally parallel to the direction of the wave. Primary Wave (P Wave) Secondary Wave (S Wave) Relative Speed Solid The Mercalli Intensity Scale and the Richter Magnitude Scale The Mercalli Intensity Scale (see Table 16.1 p. 452 text) assesses the intensity (damage, destruction) caused by an earthquake at a specific location. Note that the damage caused by an earthquake depends on several variables. i. The strength of the earthquake ii. The distance from the epicenter iii. The type of material on which objects sit. 5 iv. The design of buildings The Richter Scale (see Table 16.2 p. 454 text) measures the magnitude (amount of energy released) of an earthquake by measuring the amplitude of the largest wave recorded on the seismogram (see Figure 16.16 p. 453 text). To ensure that all seismic stations record the same Richter magnitude for a given earthquake, adjustments are made for the weakening of the seismic waves as the distance from the epicenter increases and for the variations in sensitivity of the seismograph. The scale is logarithmic with an increase of 1 in magnitude meaning a wave recorded on the seismogram has an amplitude which is 10 times larger. As well, each increase of 1 in magnitude means that about 32 times more energy is released by the earthquake. So an earthquake measuring 6 on the Richter scale releases 32 times as much energy as an earthquake measuring 5 on the Richter scale and 32x32 ≈ 1000 times as much energy as a magnitude 4 earthquake (see Table 16.3 p. 454 text). The two scales are different because a modest earthquake on the Richter scale could occur in a populated area with weak bedrock and poorly constructed buildings and cause enormous damage. This would cause the earthquake to receive a relatively high rating on the Mercalli scale. As well, an earthquake with a high Richter magnitude may occur far from a populated area and be felt by no one and cause no damage. This would cause it to have a very low rating on the Mercalli Scale. Activity: Complete the following table. Type of Scale Richter What is measured How is it measured Magnitude Seismographs (energy released) 6 Type of numbers used Type of scale Modified Mercalli Roman Numerals Closed (I – XII) Sample Exam Question 1. Distinguish between an earthquake’s magnitude and intensity. 2. What information do you need to calculate an earthquake's intensity? a) The difference in arrival times of P and S waves. b) The time that the ground was shaking. c) Records of 3 seismographs at different distances from the source. d) Records of observations and damages produced by the event. How to Locate the Epicenter of an Earthquake The difference in the speeds of P and S waves allows the distance to the epicenter to be determined. In general, since P waves travel faster than S waves, the farther the away the epicenter is located, the greater the time difference between when these waves are detected by the seismograph. First, several seismograms similar to the one in Figure 16.10 on p 447 of the text are used to create a travel-time graph (see Figure 16.11 p. 448 text). If you examine the seismogram on page 447, you should be able to determine that approximately 5 minutes passed between the arrival of the first P wave and the first S wave. Looking at the traveltime graph in Figure 16.11, we can then determine where the travel time difference between the S and P waves is 5 minutes. Extending a vertical line down to the distance axis gives the approximate distance to the epicenter, which is approximately 3800km in this example. You will follow this procedure in Parts 1 and 2 of Lab #7. Knowing the distance to the epicenter alone will not allow you to determine the exact location of the epicenter. You also need to know the direction. The exact location of the epicenter can be located using 7 the distance from three seismic stations. Once the distance to the epicenter is determined for the three stations, a circle of the correct radius is drawn around each station. The point of intersection of the three circles gives the exact location of the epicenter (see Figure 16.12 p. 449 text). You will do this in Lab #5. Sample Exam Question Why is a seismic record from 3 locations required to find the location of the epicenter? Core Lab #5: Locating an Earthquake Epicentre Do #'s 1, 2, 4, 6, 7, 8, 9, 10, 13, 14, 15, 17, 22 p. 469 text. Read Chapter Summary items 1-4, 6, 7 p. 468 text. Read pp. 475-477, 483-485 for next day 8